The importance of reducing signal noise in a fetal ECG signal is described in WO-A-00/36975. Changes in the ST interval of the fetal Electrode Cardiogram (ECG) are known to reflect the stress of the fetal heart during labour. Lack of oxygen may cause an ST rise with increased ST segment and T-wave amplitude, the appearance of so-called bi-phasic ST changes with an ST segment having a negative slope, and the appearance of negative T-waves. The electrode is mounted on the longitudinal axis of the fetus in order to be sensitive to the ECG wave form changes, usually on the scalp of the fetus as this is the part which should present first.
Fetal scalp electrodes generally tend to be of the spiral electrode type, and these have not changed significantly since the beginning of the 1970's. An example is shown in U.S. Pat. No. 3,827,428. It comprises a cylindrical body, which is approximately 1 cm long and 0.5 cm in diameter, with a spiral electrode exiting from one end, usually in the form of a single helix requiring a 360° turn for full insertion. The height of the spiral electrode is chosen so that it should not penetrate into the parietal bone of the fetus whilst providing an electrode that is as long as possible to record a strong signal. In some arrangements, the electrode may have two helical prongs and require only a turn through 180° in order to locate the electrode properly. At the other end of the cylindrical housing, a plate electrode extends axially and projects into the amniotic fluid to take a reference or ground potential. Using a slot in the end of an applicator tube, this plate electrode can be driven to screw the device into the fetus. In addition to double helix designs, other designs such as the Copeland style electrode are available, but the single helix design is by far the most widely used and is the fetal electrode that provides the most consistent ECG trace.
There are documented clinical problems associated with the design of the fetal spiral electrode. One is that fetal scalp hair and tissue becomes entrapped between the electrode wire and the plastic body of the electrode hub, which can make it both difficult to attach properly and to remove.
Another associated problem is that the fetal electrode may unscrew itself due to the pitch of the spiral wire and the slippery surface of the hub, especially in the presence of vernix caseosa on the fetal scalp and amniotic fluid. One solution proposed in U.S. Pat. No. 5,222,498 suggests preventing spontaneous unwinding by incorporating barbs on the hub surface. However, these are generally disliked and are likely to damage the fetal epidermis during electrode removal.
CA-A-2128766 provides a solution in part to the problem. In that device, the spiral electrode is sunken into a recess within the end of the cylindrical housing. The recess helps to trap tissue and thereby prevent unwinding of the fetal electrode. However, in practice there are still problems with the retention of the fetal electrode, as well as difficulties within the manufacturing phase through complicated and expensive tooling.
Midwives have also described vaginal lacerations on patients, which they believe to originate from the sharp reference part of the fetal scalp electrode. The reference electrode is usually produced by punching, which can leave sharp edges on one side even after the finishing process. The current shape of the electrode hub also provides a long lever which can be overturned during labour, resulting in the sharp spiral tip being pressed out from the fetal epidermis and possible vaginal lacerations.
The fetal scalp electrode ECG (fECG) signal quality is highly dependent on the penetration of the spiral wire electrode into the fetal epidermis. It has been found that if any part of the spiral wire should be exposed to the environment outside the fetal epidermis then this can greatly affect signal quality. This is especially noticeable in the fECG-trace when the scalp electrode spontaneously unwinds. It can lead to a substantial loss of signal amplitude, for example, a loss of 80% of the signal amplitude (i.e. a reduction to one fifth of the signal amplitude) has been observed in real recordings.
FIG. 1 shows the trace from a real case where a poorly applied fetal electrode later fell off the scalp of the fetus. The traces in the far left box show the initial averaged ECG complexes and the corresponding CTG (CardioTocoGram) trace. With the poor application, the QRS-amplitude is around 100 μV. The boxes on the right show the situation about one hour later when the electrode has been correctly re-applied. As can be seen, the QRS-amplitude is then instead 290 μV. Tests have also been conducted on adults in a bath tub filled with water containing 0.9% NaCl, showing an initial loss in the fECG signal amplitude of a factor of two for the first 90° of unwinding of the spiral electrode. The resulting ECG traces for a near perfectly applied fetal electrode, one which has been unscrewed a quarter of a turn, and one which has been unscrewed half of a turn, are shown in FIG. 2. The total drop in QRS-amplitude was from 210 μV to 70 μV when the fetal scalp electrode was unwound by half a turn. At this point, part of the spiral electrode would be visible to an observer on inspection.
A major factor in determining whether a fetal electrode is applied well, which in turn helps to keep it from unwinding, is that enough, but not too much torque is used during the application. This can be difficult to achieve in practice because the drive tubes of the currently available fetal scalp electrodes are quite soft in order to protect the fetus from the fetal electrode being overtightened. This softness results in poor tactile feedback to the midwife which in turn can lead to uncertainty in knowing whether the electrode is properly attached. Despite this and the problems mentioned above, the design of the electrode assemblies that are in every day use have hardly changed from the arrangement shown in U.S. Pat. No. 3,827,428.
A few more complex solutions have been proposed for the applicating devices, incorporating a number of mechanical elements in order to limit the maximum torque that is applied during insertion. For example, in U.S. Pat. No. 4,577,635 the electrode assembly is provided with a torque limiting device in the form of a helical spring which is rotatable with the fetal electrode at low torque but which is arranged to disengage from frictional driving contact at the torque limit. This has allowed for a stiffer drive tube. U.S. Pat. No. 5,388,579 is a further example where attempts have been made to provide a clutch within the drive mechanism in order to be able to limit the amount of torque that is applied and to allow the use of a stiffer drive rod.
When the fetal electrode comes loose, in addition to loss of the fECG signal amplitude there is also an increase in the competing low frequency (base line) noise through the exposed spiral wire electrode being subjected to relative feto-maternal movement and varying amounts of amniotic fluid. In U.S. Pat. No. 5,183,043, it is recognised that noise originating from relative feto-maternal movement can be reduced and low frequency electrical activity from the fetus can be picked up with better accuracy by providing a chloridized silver coating on a Copeland-type electrode, allowing consistent waveforms (P, QRS and T waves) of the cardiac cycle to be more easily identified. A coating of a non-conducting varnish may be provided on regions without the chloridized silver layer. This would include the tip of the Copeland electrode, which in use projects outside the fetal epidermis and therefore must be isolated in order to obtain full fECG signal amplitude. It has been shown that the resulting amplitude of a Copeland electrode can be as little as half of that of a conventional single helix electrode.
Although U.S. Pat. No. 5,183,043 suggests that a chloridized silver layer may be used in conjunction with other types of electrode, further experiments using such a chloridized silver coating on a traditional helix electrode has showed no difference compared with a plain stainless steel electrode, since the main source of noise, which originates from muscular activity, can only be made negligible by increasing the amplitude of the ECG signal if the surface of the stainless steel has been subjected to a process of passivation. Here, passivation is the chemical treatment of a stainless steel with a mild oxidant, such as a nitric acid solution, for the purposes of enhancing the spontaneous formation of a protective passive film. For fECG signals this means that unwanted surface corrosion dependent electrical noise is minimized due to the formation of a thin transparent film of inert chromic oxide.
The increased fECG signal amplitude from a spiral electrode which has been totally embedded in the fetal epidermis, is attributed to the difference in the electric volume conductivity in the interface of the fetal epidermis and the surrounding maternal tissue and amniotic fluid. The lower electrical volume conductivity present in this interface results in an increased ECG voltage potential, if measured exclusively within the fetal epidermis. In addition, the presence of the isolating vernix caseosa on the fetal head contributes to this effect. As fetal tissues and the vernix caseosa have the lowest volume conductivity and the amniotic fluid has the highest, it is important to keep the scalp electrode spiral entirely separated from the amniotic fluid.
Attempts have been made to provide a fetal scalp electrode design which keeps the spiral electrode separated from the amniotic fluid, for example, as shown in U.S. Pat. No. 7,016,716. However, the relatively large diameter of the sensor makes it more difficult to apply in the first stage of labour as well as increasing manufacturing costs, and therefore this type of fetal electrode has not been adopted for common use.
Thus, it can be seen that there is an overriding technical problem of how to improve the design of fetal scalp electrodes so that the ECG trace quality can be optimised.